Switching activity reduction through retiming

ABSTRACT

Aspects of the invention relate to techniques for using retiming to reduce circuit switching activity. Switching activity values at output ports of circuit elements of a circuit design are first computed based on switching activity values at input ports of the circuit elements and scaling factors associated with the circuit elements. Based on the switching activity values at the output ports of the circuit elements, one or more regions of the circuit design for retiming are identified. Retiming location information is then determined for the one or more regions. Finally, the identified one or more regions are then retimed to reduce switching activity based on the retiming location information.

FIELD OF THE INVENTION

The present invention relates to the field of circuit design technology.Various implementations of the invention may be particularly useful forreducing switching activity and thus power consumption of a circuitdesign.

BACKGROUND OF THE INVENTION

Low power consumption has become one of the most important features ofcurrent electronic systems. For popular consumer electronic applicationssuch as mobile smart phones and tablets, low power consumption may bethe tightest constraint in the design. Consequently, various techniquesand tools that enable tight power consumption control during design havebeen developed. Many of these techniques and tools rely on changing thearchitecture or the technology of the circuit. But once thesearchitectural or technological improvements have been made, it is theswitching of the logic that will affect the power consumption.

One cause of the switching activity is signal propagation delays. FIG. 1illustrates an example about how the switching activity can be caused bygate delays. The circuit 100 shown in the figure has a NOT gate 110 andan AND gate 120. The signal waveforms at nodes A, B and C are shownbelow. As can be seen from the waveforms, due to the delay of switchingfrom high to low by the output of the NOT gate 110, the AND gate 120outputs an unwanted pulse. In addition to the gate delay, wires can alsocause a propagation delay. Wires have an approximate propagation delayof 1 ns for every 6 inches (15 cm) of length, while logic gates can havepropagation delays ranging from more than 10 ns down to the picosecondrange, depending on the technology being used.

The switching activity can be propagated and accumulated in a circuit,causing high cumulative switching activity in a portion of a circuit.Each node in the portion of the circuit has a switching activity value.The sum of the switching activity values for all of the nodes gives riseto a cumulative switching activity value, which can be extremely high.FIG. 2 illustrates an XOR tree commonly used in arithmetic operationssuch as addition and multiplication. The XOR tree 200 is formed by aplurality of three-input XOR gates. For a three-input XOR gate, theoutput is 0 if the initial state of the input signal is 101. On a givenclock edge, the input signal changes to 010 and then the output of theXOR gate should switch to 1. Suppose, however, signals for the threeinput ports of the XOR gate arrive at different times under a sequenceof “101-111-011-010”. The output will go through a sequence of“0-1-0-1”. Two unwanted transitions are thus caused by the delay. Theunwanted transitions and propagation delays can cause more unwantedtransitions at the output ports of the downstream XOR gates. The morestages of the XOR tree, the more unwanted transitions. Thus, a big anddeep XOR tree can have a cumulative switching activity value close to amillion.

Retiming has been proposed in some research papers as a technique tominimize the switching activity caused by the propagation delay. Thetechnique is based on the observation that the output of a circuit stateelement such as a flip flop and a latch has fewer transitions than theinput of the circuit state element. In particular, unwanted transitionscan be blocked. To determine where to reposition circuit state elements,those research papers discuss employing some models to estimate averageswitching activity. While reasonably accurate, these models requiresignificant computing resources and may not be feasible for applicationsto current large circuit designs. More efficient techniques aredesirable.

BRIEF SUMMARY OF THE INVENTION

Aspects of the invention relate to techniques for using retiming toreduce circuit switching activity. With various embodiments of theinvention, switching activity values at output ports of circuit elementsof a circuit design are first computed based on switching activityvalues at input ports of the circuit elements and scaling factorsassociated with the circuit elements. The scaling factors representeffects of signal changes at the input ports on signal changes at theoutput ports. For circuit elements like logic gates, the scaling factormay be derived based on the logic gate types and the number of the inputports. The computation of switching activity values comprisescalculating weighted sums of the switching activity values at the inputports of each of the circuit elements using the scaling factors asrelative weights.

To account for a linearization effect, an upper threshold value derivedbased on signal propagation delay and slew rate may be assigned to theswitching activity value at the output port of a circuit element if theweighted sum of the switching activity values at the input ports of thecircuit element is greater than the upper threshold value.

To account for or model for potential worst case condition/scenario, aswitching activity value at one of the input ports of a circuit elementmay be assigned to the switching activity value at the output port ofthe circuit element if the weighted sum of the switching activity valuesat the input ports of the circuit element is less than the switchingactivity value at the one of the input ports.

Based on the switching activity values at the output ports of thecircuit elements, one or more regions of the circuit design for retimingare identified. The identification processes may comprise derivingcumulative switching activity values for regions in the circuit design.The cumulative switching activity value of a region is the sum of allswitching activity values in the region.

Next, retiming location information for the one or more regions isdetermined based on the switching activity values at the output ports ofthe circuit elements. The retiming location information comprisesinformation of one or more circuit nodes for placing circuit stateelements to reduce switching activity in each of the one or moreregions. The retiming location information may comprise information ofswitching activity relaxing point nodes determined based on thecumulative switching activity values.

Finally, the identified one or more regions may be retimed based on theretiming location information. The operation of retiming comprisesrepositioning existing circuit state elements such as flip flops andlatches. Additionally, the operation of retiming may comprise addingcircuit state elements triggered at a clock edge inverse to that atwhich the existing circuit state elements are triggered (i.e.,inverse-edge-triggered circuit state elements).

Certain inventive aspects are set out in the accompanying independentand dependent claims. Features from the dependent claims may be combinedwith features of the independent claims and with features of otherdependent claims as appropriate and not merely as explicitly set out inthe claims.

Certain objects and advantages of various inventive aspects have beendescribed herein above. Of course, it is to be understood that notnecessarily all such objects or advantages may be achieved in accordancewith any particular embodiment of the invention. Thus, for example,those skilled in the art will recognize that the invention may beembodied or carried out in a manner that achieves or optimizes oneadvantage or group of advantages as taught herein without necessarilyachieving other objects or advantages as may be taught or suggestedherein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example about how gate delays can lead toswitching activity.

FIG. 2 illustrates an XOR tree commonly used in arithmetic operationssuch as addition and multiplication.

FIG. 3 shows an illustrative example of such a programmable computersystem.

FIG. 4 illustrates an example of a retiming tool according to variousembodiments of the invention.

FIG. 5 illustrates a flowchart showing a process of retiming to reduceswitching activity that may be implemented according to various examplesof the invention.

FIG. 6 illustrates an example of a ten-level XOR tree.

FIG. 7 illustrates a region with high switching activity in a circuitdesign.

FIG. 8 illustrates an example of retiming the region 700 to improvecircuit speed (prior art).

FIG. 9 illustrates an example of retiming the region 700 to reduceswitching activity according to various embodiments of the invention.

FIG. 10 illustrates an example of repositioning and adding circuit stateelements according to various embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

General Considerations

Various aspects of the present invention relate to techniques for usingretiming to reduce circuit switching activity. In the followingdescription, numerous details are set forth for the purpose ofexplanation. However, one of ordinary skill in the art will realize thatthe invention may be practiced without the use of these specificdetails. In other instances, well-known features have not been describedin details to avoid obscuring the present invention.

Some of the techniques described herein can be implemented in softwareinstructions stored on a computer-readable medium, software instructionsexecuted on a computer, or some combination of both. Some of thedisclosed techniques, for example, can be implemented as part of anelectronic design automation (EDA) tool. Such methods can be executed ona single computer or on networked computers.

Although the operations of the disclosed methods are described in aparticular sequential order for convenient presentation, it should beunderstood that this manner of description encompasses rearrangements,unless a particular ordering is required by specific language set forthbelow. For example, operations described sequentially may in some casesbe rearranged or performed concurrently. Moreover, for the sake ofsimplicity, the disclosed flow charts and block diagrams typically donot show the various ways in which particular methods can be used inconjunction with other methods. Additionally, the detailed descriptionsometimes uses terms like “compute” and “identify” to describe thedisclosed methods. Such terms are high-level abstractions of the actualoperations that are performed. The actual operations that correspond tothese terms will vary depending on the particular implementation and arereadily discernible by one of ordinary skill in the art.

Also, as used herein, the term “design” is intended to encompass datadescribing an entire integrated circuit device. This term also isintended to encompass a smaller group of data describing one or morecomponents of an entire device, however, such as a portion of anintegrated circuit device. Still further, the term “design” also isintended to encompass data describing more than one microdevice, such asdata to be used to form multiple microdevices on a single wafer.

Illustrative Operating Environment

The execution of various electronic design automation processesaccording to embodiments of the invention may be implemented usingcomputer-executable software instructions executed by one or moreprogrammable computing devices. Because these embodiments of theinvention may be implemented using software instructions, the componentsand operation of a generic programmable computer system on which variousembodiments of the invention may be employed will first be described.

FIG. 3 shows an illustrative example of such a programmable computersystem (a computing device 301). As seen in this figure, the computingdevice 301 includes a computing unit 303 with a processing unit 305 anda system memory 307. The processing unit 305 may be any type ofprogrammable electronic device for executing software instructions, butwill conventionally be a microprocessor. The system memory 307 mayinclude both a read-only memory (ROM) 309 and a random access memory(RAM) 311. As will be appreciated by those of ordinary skill in the art,both the read-only memory (ROM) 309 and the random access memory (RAM)311 may store software instructions for execution by the processing unit305.

The processing unit 305 and the system memory 307 are connected, eitherdirectly or indirectly, through a bus 313 or alternate communicationstructure, to one or more peripheral devices. For example, theprocessing unit 305 or the system memory 307 may be directly orindirectly connected to one or more additional memory storage devices,such as a “hard” magnetic disk drive 315, a removable magnetic diskdrive 317, an optical disk drive 319, or a flash memory card 321. Theprocessing unit 305 and the system memory 307 also may be directly orindirectly connected to one or more input devices 323 and one or moreoutput devices 325. The input devices 323 may include, for example, akeyboard, a pointing device (such as a mouse, touchpad, stylus,trackball, or joystick), a scanner, a camera, and a microphone. Theoutput devices 325 may include, for example, a monitor display, aprinter and speakers. With various examples of the computer 301, one ormore of the peripheral devices 315-325 may be internally housed with thecomputing unit 303. Alternately, one or more of the peripheral devices315-325 may be external to the housing for the computing unit 303 andconnected to the bus 313 through, for example, a Universal Serial Bus(USB) connection.

With some implementations, the computing unit 303 may be directly orindirectly connected to one or more network interfaces 327 forcommunicating with other devices making up a network. The networkinterface 327 translates data and control signals from the computingunit 303 into network messages according to one or more communicationprotocols, such as the transmission control protocol (TCP) and theInternet protocol (IP). Also, the interface 327 may employ any suitableconnection agent (or combination of agents) for connecting to a network,including, for example, a wireless transceiver, a modem, or an Ethernetconnection. Such network interfaces and protocols are well known in theart, and thus will not be discussed here in more detail.

It should be appreciated that the computer 301 is illustrated as anexample only, and it is not intended to be limiting. Various embodimentsof the invention may be implemented using one or more computing devicesthat include the components of the computer 301 illustrated in FIG. 3,which include only a subset of the components illustrated in FIG. 3, orwhich include an alternate combination of components, includingcomponents that are not shown in FIG. 3. For example, variousembodiments of the invention may be implemented using a multi-processorcomputer, a plurality of single and/or multiprocessor computers arrangedinto a network, or some combination of both.

Retiming Tools and Methods

FIG. 4 illustrates an example of a retiming tool according to variousembodiments of the invention. As seen in the figure, the retiming tool400 includes three units: a switching activity computation unit 410, aretiming region identification unit 420 and a retiming locationdetermination unit 430. Some implementations of the retiming tool 400may cooperate with (or incorporate) one or more of, a retiming unit 440,an input database 405 and an output database 435.

As will be discussed in more detail below, the switching activitycomputation unit 410 computes switching activity values. Based on theswitching activity values, the retiming region identification unit 420identifies one or more regions of the circuit design that have highswitching activity. The retiming location determination unit 430 thendetermines retiming location information for the one or more regionsbased on the switching activity values. Finally, the retiming unit 440retimes the one or more identified regions based on the retiminglocation information.

As previously noted, various examples of the invention may beimplemented by a computing system, such as the computing systemillustrated in FIG. 3. Accordingly, one or more of the switchingactivity computation unit 410, the retiming region identification unit420, the retiming location determination unit 430 and the retiming unit440 may be implemented by executing programming instructions on one ormore processors in a computing system such as the computing systemillustrated in FIG. 3. Correspondingly, some other embodiments of theinvention may be implemented by software instructions, stored on anon-transitory computer-readable medium, for instructing one or moreprogrammable computers/computer systems to perform the functions of oneor more of the switching activity computation unit 410, the retimingregion identification unit 420, the retiming location determination unit430 and the retiming unit 440. As used herein, the term “non-transitorycomputer-readable medium” refers to computer-readable medium that arecapable of storing data for future retrieval, and not propagatingelectro-magnetic waves. The non-transitory computer-readable medium maybe, for example, a magnetic storage device, an optical storage device, a“punched” surface type device, or a solid state storage device.

It also should be appreciated that, while the switching activitycomputation unit 410, the retiming region identification unit 420, theretiming location determination unit 430 and the retiming unit 440 areshown as separate units in FIG. 4, a single servant computer (or asingle processor within a master computer) may be used to implement twoor more of these units at different times, or components of two or moreof these units at different times.

With various examples of the invention, the input database 405 and theoutput database 435 may be implemented using any suitable computerreadable storage device. That is, either of the input database 405 andthe output database 435 may be implemented using any combination ofcomputer readable storage devices including, for example, microcircuitmemory devices such as read-write memory (RAM), read-only memory (ROM),electronically erasable and programmable read-only memory (EEPROM) orflash memory microcircuit devices, CD-ROM disks, digital video disks(DVD), or other optical storage devices. The computer readable storagedevices may also include magnetic cassettes, magnetic tapes, magneticdisks or other magnetic storage devices, punched media, holographicstorage devices, or any other non-transitory storage medium that can beused to store desired information. While the input database 405 and theoutput database 435 are shown as separate units in FIG. 4, a single datastorage medium may be used to implement some or all of these databases.

FIG. 5 illustrates a flowchart showing a process of retiming to reduceswitching activity that may be implemented according to various examplesof the invention. For ease of understanding, methods of retiming toreduce switching activity that may be employed according to variousembodiments of the invention will be described with reference to theretiming tool 400 illustrated in FIG. 4 and the flow chart 500 in FIG.5. It should be appreciated, however, that alternate implementations ofa retiming tool may be used to perform the method of retiming to reduceswitching activity shown in the flow chart 500 according to variousembodiments of the invention. In addition, it should be appreciated thatimplementations of the retiming tool 400 may be employed to implementmethods of retiming to reduce switching activity according to differentembodiments of the invention other than the one illustrated by the flowchart 50 in FIG. 5.

Initially, in operation 510, the switching activity computation unit 410computes switching activity values at output ports of circuit elementsof a circuit design based on switching activity values at input ports ofthe circuit elements and scaling factors associated with the circuitelements. As a starting point, the switching activity values at primaryinput ports may be set as 1. The switching activity value at the outputof a flip flop may also be set as 1 because the output changes only withrespect to the clock.

Each circuit element is assigned one or more scaling factorsrepresenting the effect of signal changes at the input ports on signalchanges at the output ports. The scaling factor may be derived based onthe type of the circuit element and the number of the input ports. For atwo-input XOR gate, for example, a value change (switching activity) atone input port will lead to a value change (switching activity) at theoutput port no matter whether the signal at the other input port is alogic 1 or 0. The scaling factor of the two-input XOR gate may thus beset as 1. On the other hand, a value change at one input port of afour-input AND gate will result in a value change at the output port ofthe AND gate only when all three other input ports are held to a logicvalue of 1. This represents one out of eight possible input valuecombinations. Accordingly, the scaling factor of the four-input AND gatemay be set as ⅛.

The switching activity value at the output port of a circuit elementsuch as a logic gate may then be calculated by obtaining a weighted sumof the switching activity values at the input ports using the scalingfactor as relative weight. Accordingly, the two-input XOR gate has aswitching activity value at its output port of (X+Y), while thefour-input AND gate has a switching activity value of (⅛*W+⅛*X+⅛*Y+⅛*Z)at its output port. Here, W, X, Y and Z represent the input activityvalues.

With various implementations of the invention, an upper threshold valuemay be set for the switching activity value at the output of a circuitelement. The upper threshold value is used to account for alinearization effect—the switching activity also depends on the wirelength and slew rate for 0-1 and 1-0 transitions. This value can't becomputed precisely but can be approximated as some linear function ofthe maximum depth path to reach the signal starting at state elements orinputs. Given a bounded slew rate, there is a finite minimum time forthe signal to go up and then down. The switching activity value may bebounded by the time for the signal to reach a stable value divided bythe average of rise and fall time. If a minimum delay is known for thefastest path to reach the signal from an input or state element then thebound may be further improved to be (max delay−min delay)/Averageup/down time OR (max delay−min delay)*2/(up time+down time).

To account or model for potential worst case condition/scenario, someembodiments of the invention may assign the switching activity value ofan input port of a circuit element to the switching activity value of anoutput port of the circuit element if the input port activity valueexceeds the value which would be computed for the output port usingother means. For example, with low scaling factors, the weighted sum maybe smaller than the switching activity values at one or more input portsof a circuit element. The circuit element may serve as a buffer for theinput with the maximum switching activity value. Accordingly, themaximum switching activity value of any input, rather than the weightedsum, may be used as the switching activity value at the output port.

The following is an example of a pseudo code for computing switchingactivity values:

// SA(u): denotes the Score Activity of a node ‘u’. // MaxDepth(u): MaxDepth of the node ‘u’ in the design from the primary input/stateelement. // MinDepth(u): Min Depth of the node ‘u’ in the design fromthe primary input/state element.  const cTransitionsPerMFCDelay = 25; computeScoreActivityForANode (v) { outScore = 0; maxInScore = 0;maxDepth = −1; minDepth= 1000000; // Compute the activity by traversingall its fan-ins.. for each node ‘u’ such that there is any edge from uto v { inScore = SA(u); // inScaleFactor is the scaling factor of inputnode ‘u’ // for the corresponding node ‘v’. outScore += inScore *inScaleFactor; if (inScore > maxInScore) { maxInScore = inScore; } depth= MaxDepth(u); if (depth > maxDepth) { maxDepth = depth; } mdepth =MinDepth(u); if (mdepth < minDepth) { minDepth = mdepth; } } /** ToHandle the non-linearity in the algorithm **/ // maxInScore models thefact that in the worse case even a LUT with // a low activity equationmay be in a state that treats an input as // a buffer as long as this istrue for some state of the other // inputs. So if the weighted sum ofinput scores is less than the // maximum score on some input, model asif the LUT is in a state // that transmits all changes on this input.However, add a small // decay term so that in a chain of low activityLUTs there is a // decrease in score. maxInScore = maxInScore * 0.95;Float64 depth_limit = cTransitionsPerMFCDelay * (maxDepth + 1.0 −(2.0*(minDepth+1.0)/3.0)); /** For the Lineralization effect **/ if(outScore > depth_limit) { outScore = depth_limit; } if (outScore >maxInScore) { maxInScore = outScore; } SA(v) = maxInScore; MaxDepth(v) =maxDepth+1; MinDepth(v) = minDepth+1; }

Based on the computed switching activity values, in operation 520, theretiming region identification unit 420 identifies one or more regionsof the circuit design that have high switching activity. Theidentification processes may comprise deriving cumulative switchingactivity values for regions in the circuit design. The cumulativeswitching activity value of a region is the sum of all switchingactivity values in the region. FIG. 6 illustrates an example of aten-level XOR tree. Table 1 lists the switching activity value for eachXOR gate at each level and the number of nodes at each level that may bederived by an algorithm similar to the pseudo code shown in operation510. Using the data in Table 1, the cumulative switching activity valuefor the XOR tree is calculated to be 2.86×10⁵. While this number mayreflect the worst case scenario, it indicates that the switchingactivity for this region is too high and that a significant amount ofpower may be consumed by the switching activity.

TABLE 1 Level 0 1 2 3 4 5 6 7 8 9 10 Activity Values 1  3  9  27 41.6 5058.3 66.6 75 83.3 91.7 Num. Nodes 3¹⁰ 3⁹ 3⁸   3⁷  3⁶   3⁵  3⁴  3³   3² 3¹  3⁰

Using the cumulative switching activity values, the retiming regionidentification unit 420 then identifies one or more regions of thecircuit design for retiming. For example, the retiming regionidentification unit 420 may select a cumulative switching activitythreshold to identify regions for retiming. Any regions in the circuitwith cumulative switching activity values above the threshold will beselected to be processed by the following retiming operation. Thethreshold may be determined by heuristics.

Next, in operation 530, the retiming location determination unit 430determines retiming location information for the one or more regionsbased on the switching activity values at the output ports of thecircuit elements. The retiming location information comprisesinformation of one or more circuit nodes for placing circuit stateelements to reduce switching activity in each of the one or moreregions.

A conventional operation of retiming comprises repositioning the circuitstate elements such as flip flops and latches in a sequential circuitwhile maintaining its external functional behavior. Retiming has beenwidely used to minimize the delay of the longest path, thus allowing thecircuit to operate at higher clock speeds. This retiming applicationrelies on the fact that delay varies linearly under retiming.Unfortunately, it is not so with switching activity.

To reduce switching activity, circuit nodes for placing circuit stateelements are determined differently by the retiming locationdetermination unit 430. With some implementation of the invention, thesecircuit nodes are referred to as switching activity relaxing pointnodes. The switching activity relaxing point nodes are determined basedon the cumulative switching activity values derived in the operation520. The following is an example of a pseudo code for determiningswitching activity relaxing point nodes:

ARCheck(G, TCA) For each node v { If (v is a PI) s(v) = 0; Else s(v) =−infinity; isActivityRelaxPoint(v) = FALSE; ComputeRF(v, TCA); } For I =1 to n // n is the number of nodes. { Done = TRUE; For each node ‘v’ {// calculate the maximum Required Flop values from all its fan-in maxRF= max (RF(u)) from all u’s such that there is an edge from u to v. Foreach node ‘u’ such that there is any edge from u to v { tmp = max (s(u)−w(e)); if (RF(v) > maxRF) { // We need to pull one flop tmp++; } If(tmp > 1.0 && v is a PO) return failure; // TCA cann’t be achieved assufficient flops are not present. If (tmp > s(v)) { s(v) = tmp; done =FALSE; } } If (done == TRUE) return success } } If (done == TRUE) { /**Activity Relax Points are identified where flop needs to be inserted soas to achieve the target cumulative activity of the design**/ For eachnode ‘v’ in the circuit { isValuePropagated = FALSE; For each node ‘u’such there is an edge from u to v { If ((s(u) − w(e)) == s(v)) {isValuePropagated = TRUE; break; } } If (isValuePropagated == FALSE) {isActivityRelaxPoint(v) = TRUE; } } } ComputeRF(v, TCA) {  Activity =FCA(v); NumFFReqd = 0; While (Activity >= TCA) { Activity =Activity/TCA; NumFFReqd++; } return NumFFReqd;   }

It should be noted that the algorithm shown above for determiningswitching activity relaxing point nodes is only one example. It is notintended to be limiting. Other methods for determining circuit nodes forplacing circuit state elements may be implemented. For example, afterdetermining the first switching activity relaxing point node, theswitching activity values may be re-calculated by setting switchingactivity values at the output ports of the flip flops for the firstswitching activity relaxing point node as 1.

After the circuit nodes for placing circuit state elements aredetermined, the retiming unit 440 may, in operation 540, reposition thecircuit state elements to or near these nodes.

FIGS. 7-9 uses an example to illustrate and compare the retiming forimproving circuit speed and the retiming for reducing switchingactivity. In FIG. 7, a region of a circuit design with high switchingactivity (a region 700) is illustrated. The region 700 comprises a10-level XOR tree 710 similar to the one shown in FIG. 6. As discussedpreviously, the cumulative switching activity value of the 10-level XORtree alone may reach 2.86×10⁵. Also included in the region 700 is an ORchain 720 and two flip flops 730 and 740.

FIG. 8 illustrates an example of retiming the region 700 to improvecircuit speed. To reduce the path delay, the flip flops 730 and 740 arerepositioned at the 20^(th) level (10 levels of XOR gates+10 levels ofOR gates) and the 40^(th) level (20 levels of OR gates), respectively.In effect, the region is divided into three portions ((710+810), 820,and 830) with equal delay paths by the flip flops. This is because, asnoted previously, delay varies linearly under retiming.

FIG. 9 illustrates an example of retiming the region 700 to reduceswitching activity according to various embodiments of the invention.The cumulative switching activity values (FCA) and the switchingactivity relaxing point nodes indicated by RF are listed in Table 2:

TABLE 2 Level 0 1 2 3 4 5 6 7 8 9 10 FCA 1 6 27 108 365 1146 3499 1056331763 95372 2.86 × 10⁵ RF 0 0 0 1 1 1 1 2 2 2 2

The FCA value is obtained by using the equation: FCA(v)=switchingactivity value at v+3*FCA(v−1). The RF(v) is obtained by using thealgorithm shown above (the subroutine ComputeRF(v, TCA)), assuming thetarget cumulative switching activity value between neighboring flipflops (TCA) is 102. As Table 2 shows, the RF(v) changes values at the3^(rd) level and the 7^(th) level. The switching activity relaxing pointnodes may be set at these nodes. To make it symmetric, the flip flopsare repositions at the 3^(rd) and 6^(th) levels, as illustrated in FIG.9. After the retiming, the accumulative switching activity value for thewhole region is 284, significantly smaller than 2.86×10⁵.

In addition to repositioning circuit state elements, the retiming unit440 may also add inverse-edge-triggered circuit state elements ifrepositioning existing state elements is not sufficient to meet thetarget cumulative switching activity value. FIG. 10 illustrates anexample of repositioning and adding circuit state elements according tovarious embodiments of the invention. The circuit in FIG. 10 is the sameas the circuit in FIG. 9 except there is only one existing pipe-lineflip flop 1010 for retiming. It is obvious from Table 2 thatrepositioning the existing flip flop 1010 cannot meet the targetcumulative switching activity value of 102. If devices in the fan-outcone of this original flip flop are all triggered on the same clock edgeas this pipe-line one, negative-edge-triggered flip flops may be addedafter the repositioned flip flops. This addition will not change thefunctionality of the circuit design. In FIG. 10, the existing flip flop1010 is repositioned at the 3^(rd) level of the XOR tree and thenegative-edge-triggered flip flop 1020 is added at the 6^(th) level ofthe XOR tree.

If devices in the fan-in cone of the pipe-line flip flop are triggeredon the same clock edge as the pipe-line flip flop, thenegative-edge-triggered flip flops may be added before the pipe-lineflip flop. This addition will also not change the functionality of thecircuit design.

CONCLUSION

While the invention has been described with respect to specific examplesincluding presently preferred modes of carrying out the invention, thoseskilled in the art will appreciate that there are numerous variationsand permutations of the above described systems and techniques that fallwithin the spirit and scope of the invention as set forth in theappended claims. For example, while specific terminology has beenemployed above to refer to electronic design automation processes, itshould be appreciated that various examples of the invention may beimplemented using any desired combination of electronic designautomation processes.

What is claimed is:
 1. A method, executed by at least one processor of acomputer, comprising: computing switching activity values at outputports of circuit elements of a circuit design based on switchingactivity values at input ports of the circuit elements and scalingfactors associated with the circuit elements, the scaling factorsrepresenting effects of signal changes at the input ports on signalchanges at the output ports, the computing comprising calculatingweighted sums of the switching activity values at the input ports ofeach of the circuit elements using the scaling factors as relativeweights; identifying one or more regions of the circuit design forretiming based on the switching activity values at the output ports ofthe circuit elements; and determining retiming location information forthe one or more regions based on the switching activity values at theoutput ports of the circuit elements, the retiming location informationcomprising information of one or more circuit nodes for placing circuitstate elements to reduce switching activity in each of the one or moreregions.
 2. The method recited in claim 1, further comprising: retimingthe one or more regions based on the retiming location information. 3.The method recited in claim 2, wherein the retiming comprises:repositioning circuit state elements in the one or more regions.
 4. Themethod recited in claim 3, wherein the retiming further comprises:adding inverse-edge-triggered circuit state elements.
 5. The methodrecited in claim 1, wherein the computing comprises: assigning aswitching activity value at one of the input ports of a circuit elementto the switching activity value at the output port of the circuitelement if the weighted sum of the switching activity values at theinput ports of the circuit element is less than the switching activityvalue at the one of the input ports.
 6. The method recited in claim 1,wherein the computing comprises: assigning an upper threshold value tothe switching activity value at the output port of a circuit element ifthe weighted sum of the switching activity values at the input ports ofthe circuit element is greater than the upper threshold value, the upperthreshold value being derived based on signal propagation delay and slewrate.
 7. The method recited in claim 1, wherein the identifyingcomprises: deriving cumulative switching activity values for regions inthe circuit design based on the switching activity values at outputports of circuit elements in each of the regions.
 8. The method recitedin claim 7, wherein the retiming location information comprisesinformation of switching activity relaxing point nodes determined basedon the cumulative switching activity values.
 9. The method recited inclaim 8, further comprising: repositioning circuit state elements in theone or more regions to or near the switching activity relaxing pointnodes.
 10. One or more non-transitory computer-readable media storingcomputer-executable instructions for causing one or more processors toperform a method, the method comprising: computing switching activityvalues at output ports of circuit elements of a circuit design based onswitching activity values at input ports of the circuit elements andscaling factors associated with the circuit elements, the scalingfactors representing effects of signal changes at the input ports onsignal changes at the output ports, the computing comprising calculatingweighted sums of the switching activity values at the input ports ofeach of the circuit elements using the scaling factors as relativeweights; identifying one or more regions of the circuit design forretiming based on the switching activity values at the output ports ofthe circuit elements; and determining retiming location information forthe one or more regions based on the switching activity values at theoutput ports of the circuit elements, the retiming location informationcomprising information of one or more circuit nodes for placing circuitstate elements to reduce switching activity in each of the one or moreregions.
 11. The one or more non-transitory computer-readable mediarecited in claim 10, wherein the method further comprises: retiming theone or more regions based on the retiming location information.
 12. Theone or more non-transitory computer-readable media recited in claim 11,wherein the retiming comprises: repositioning circuit state elements inthe one or more regions, adding inverse-edge-triggered circuit stateelements, or both.
 13. The one or more non-transitory computer-readablemedia recited in claim 10, wherein the computing comprises: assigning aswitching activity value at one of the input ports of a circuit elementto the switching activity value at the output port of the circuitelement if the weighted sum of the switching activity values at theinput ports of the circuit element is less than the switching activityvalue at the one of the input ports.
 14. The one or more non-transitorycomputer-readable media recited in claim 10, wherein the computingcomprises: assigning an upper threshold value to the switching activityvalue at the output port of a circuit element if the weighted sum of theswitching activity values at the input ports of the circuit element isgreater than the upper threshold value, the upper threshold value beingderived based on signal propagation delay and slew rate.
 15. The one ormore non-transitory computer-readable media recited in claim 10, whereinthe identifying comprises: deriving cumulative switching activity valuesfor regions in the circuit design based on the switching activity valuesat output ports of circuit elements in each of the regions.
 16. The oneor more non-transitory computer-readable media recited in claim 15,wherein the retiming location information comprises information ofswitching activity relaxing point nodes determined based on thecumulative switching activity values.
 17. A system, comprising: one ormore processors, the one or more processors programmed to perform amethod, the method comprising: computing switching activity values atoutput ports of circuit elements of a circuit design based on switchingactivity values at input ports of the circuit elements and scalingfactors associated with the circuit elements, the scaling factorsrepresenting effects of signal changes at the input ports on signalchanges at the output ports, the computing comprising calculatingweighted sums of the switching activity values at the input ports ofeach of the circuit elements using the scaling factors as relativeweights; identifying one or more regions of the circuit design forretiming based on the switching activity values at the output ports ofthe circuit elements; and determining retiming location information forthe one or more regions based on the switching activity values at theoutput ports of the circuit elements, the retiming location informationcomprising information of one or more circuit nodes for placing circuitstate elements to reduce switching activity in each of the one or moreregions.
 18. The system recited in claim 17, wherein the method furthercomprises: retiming the one or more regions based on the retiminglocation information.
 19. The system recited in claim 18, wherein theretiming comprises: repositioning circuit state elements in the one ormore regions, adding inverse-edge-triggered circuit state elements, orboth.
 20. The system recited in claim 17, wherein the computingcomprises: assigning a switching activity value at one of the inputports of a circuit element to the switching activity value at the outputport of the circuit element if the weighted sum of the switchingactivity values at the input ports of the circuit element is less thanthe switching activity value at the one of the input ports.
 21. Thesystem recited in claim 17, wherein the computing comprises: assigningan upper threshold value to the switching activity value at the outputport of a circuit element if the weighted sum of the switching activityvalues at the input ports of the circuit element is greater than theupper threshold value, the upper threshold value being derived based onsignal propagation delay and slew rate.
 22. The system recited in claim17, wherein the identifying comprises: deriving cumulative switchingactivity values for regions in the circuit design based on the switchingactivity values at output ports of circuit elements in each of theregions.
 23. The system recited in claim 22, wherein the retiminglocation information comprises information of switching activityrelaxing point nodes determined based on the cumulative switchingactivity values.